Method and system for enhancing oil recovery (EOR) by injecting treated water into an oil bearing formation

ABSTRACT

A process and a system for enhancing recovery of oil from an oil-bearing formation are provided in which water having a total dissolved solids content is filtered to remove some solids in a filter assembly, the filtered water is treated to remove some ions in a capacitive deionization assembly, and the filtered deionized water is injected into the oil-bearing formation to mobilize crude oil and enhance oil recovery from the formation.

CROSS REFERENCE TO EARLIER APPLICATIONS

The present application is a Continuation of application Ser. No.13/728,646, filed Dec. 27, 2012, which claims the benefit of EuropeanPatent Application No. 11196116.5, filed Dec. 29, 2011, the entiredisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method and system for Enhancing Oil Recovery(EOR) by injecting treated water into an oil bearing formation.

Only a portion of oil present in an oil-bearing formation is recoverableas a result of the natural pressure of the formation. The oil recoveredfrom this “primary” recovery typically ranges from 5% to 35% of the oilin the formation. Enhanced oil recovery methods have been developed toincrease the amount of oil that may be recovered from an oil-bearingformation above and beyond that recovered in primary recovery.

Water-flooding, in which water is injected through an injection wellinto an oil-bearing formation to mobilize and drive oil through theformation for production from a production well, is a widely used methodof secondary recovery used to increase the amount of oil recovered froma formation beyond primary recovery. Recently, water-flooding utilizingwater having low salinity has been utilized to increase the amount ofoil recovered from a formation relative to the amount of oil recoveredin a conventional higher salinity water-flood. Low salinity water may beused in place of higher salinity water conventionally used in awater-flood in a secondary recovery, or low salinity water may be usedafter a conventional higher salinity water-flood to incrementallyincrease oil recovery over that of the initial water-flood in a tertiaryrecovery process.

Injection of low salinity water into a formation may reduce the ionicbonding of oil to the formation within pores in the formation by doublelayer expansion, leading to a reduction in the rock's adsorptioncapacity for hydrocarbons. This increases the mobility of the oil in theformation by making the surface of the pores of the formation morewater-wet and less oil-wet, permitting the mobile oil to be removed fromthe pores in which it resides and to be driven to a production well forproduction from the formation.

Low salinity water utilized in low salinity water-flooding typically hasa total dissolved solids (“TDS”) content ranging from 200 parts permillion (“ppm”) to 5000 ppm, and preferably has a TDS content rangingfrom 1000 ppm to 5000 ppm to provide adequate salinity in the water toprevent formation damage.

Frequently, the low salinity water provided for enhanced oil recovery isproduced by desalinating a source water having significantly highersalinity. Seawater is a common source water treated to provide the lowsalinity water, particularly for offshore oil recovery. Seawatertypically has a TDS content between 30000 ppm and 50000 ppm. Brackishwater, high salinity formation water produced from a formation, and highsalinity aquifer water may also be utilized as source water that may bedesalinated to provide the low salinity source water. Such water sourcesmay have a TDS content ranging from 10000 ppm to 250000 ppm.

Commonly applied technologies for desalination of water includedistillation processes, such as Multi Stage Flash, Multi EffectDistillation, Mechanical Vapour Compression and/or Thermal VapourCompression, and membrane processes, such as Reverse Osmosis (RO), NanoFiltration (NF) and/or Electrodialyses. International patent applicationWO2011/135048 of Voltea B.V. and the website www.voltea.com disclose amethod and apparatus for removal of ions from, for example, wastewaterby Capacitive De-Ionisation (CDI). More information on CDI can be foundin the scientific paper Environmental Science and Technology, vol.36/13, page 3017, 2002 and in the article “Capacitive deionization as anelectrochemical means of saving energy and delivering clean water.Comparison to present desalination practices: Will it compete?” by M. A.Anderson et al. published the Journal Electrochimica Acta55(2010)3845-3856 and at website www.elsevier.com/locate/electacta.

The latter article by M. A. Anderson et al. shows in FIG. 8 the amountof electrical work required to desalinate water with differentsalinities and concludes that under the selected conditions and atconcentrations below 5000 mg/L, CDI could be a competitive technologyeven if moderate efficiencies, from 60-70%, are attained.

The most commonly used method for desalination of water used for EORgenerally comprises a Micro Filter (MF) or Ultra Filter (UF) assemblyfor filtering solids from the water and a Reverse Osmosis (RO) assemblyor a combination of a nanofiltration assembly and a RO assembly forsubsequent water desalination. Several studies on offshore desalinationof seawater have concluded that Seawater Reverse Osmosis (SWRO) witheither conventional or membrane pre-treatment is by far the most viabledesalination method available for offshore application due to suitableweight, cost, footprint, and designed output capacities.

A proper treatment of EOR low salinity injection water is critical toprevent salinity related formation damage. If the clays that are presentin the formation are incompatible with the injection water,de-flocculation of the clays could occur. When the clays de-flocculatein the formation, the clay particles may disperse and migrate into thepore throats, resulting in formation damage. In general the injectionwater/completion fluid must have an adequate salinity (measured in totaland/or divalent cation concentration) to prevent de-flocculation offormation clays when the system is in equilibrium. Additionally theremust be enough divalent cations (i.e. Ca⁺⁺, Mg⁺⁺) present in thedisplacing fluid (e.g. injected seawater) to prevent de-flocculation ofthe formation clay during the transition from one water composition toanother.

A drawback of both distillation technologies and SWRO's, is that thetreated source water has a too high purity requiring blending withseawater or a high salinity membrane retentate stream to adjust the TDSlevel to the desired levels. Distillation and RO membrane desalinationtechnologies typically reduce the TDS content of the treated sourcewater to less than 500 ppm, often less than 200 ppm. To avoid formationdamage, a low salinity water having a TDS of from 1000 ppm to 5000 ppmis desirable, therefore, ions are typically added back to water producedby distillation or RO membrane desalination technologies for use in anEOR application, for example by blending with seawater or with a highsalinity membrane retentate stream. Further drawbacks of RO are that ROmembranes are sensitive to fouling and RO is energy-intensive.

There is a need to provide an improved and efficient seawater treatmentmethod and system for EOR, which provide treated water with purity,salinity and TDS level suitable for EOR and which therefore do notrequire subsequent re-blending with raw seawater to re-adjust the TDSlevel to a desired level, and which is less sensitive to fouling andless energy-intensive than RO.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method forEnhancing Oil Recovery (EOR) from an oil bearing formation, the methodcomprising: filtering at least some solids from a source water having atotal dissolved solids content of from 10000 ppm to 50000 ppm in afiltration assembly to produce pre-treated water; further treating thepre-treated water in a Capacitive De-Ionisation (CDI) assemblycomprising at least one flowpath for pre-treated seawater arrangedbetween a pair of oppositely charged electrodes which adsorb and therebyremove at least some ions from the pre-treated water flowing through theflowpath, thereby producing treated water with a reduced salinity andsolids content relative to the source water; and

injecting the treated water with reduced salinity and solids contentinto the formation to mobilize crude oil and enhance oil recovery.

The electrodes may comprise substantially parallel porous platescomprising carbon aerogel and/or activated carbons that are electricallycharged by a Direct Current (DC) electrical power source connected tothe plates.

The filtration assembly may comprise a microfilter assembly. Themicrofilter assembly may generate pre-treated seawater with a reducedhardness and sulphate concentration and with less than 1 parts permillion (ppm) oil and less than 1 parts per million (ppm) of TotalSuspended Solids (TSS).

The source water may had a total dissolved solids (TDS) content of from10000 ppm to 50000 ppm. The source water may be selected from the groupconsisting of seawater, brackish water, water produced from theformation, saline aquifer water, and mixtures thereof. The treated waterhas reduced salinity and reduced solids content relative to the sourcewater. The treated water may have a TDS between 1,000 and 5,000 partsper million (ppm), or from 2000 ppm to 5000 ppm.

It is believed that the above operating envelope of an NF and CDIassembly has a synergetic effect that optimizes the efficiency andperformance of the NF and CDI assembly in an unexpected manner, whichovercomes the prejudice stemming from the article by M. A. Anderson etal. that CDI would only be efficient for desalination of brackish waterwith a TDS of less than 5000 mg/Liter.

In accordance with the invention there is furthermore provided a systemfor Enhancing Oil Recovery (EOR) from an oil bearing formation, thesystem comprising:

a filtration assembly for filtering at least some solids from a sourcewater having a TDS content of from 10000 ppm to 50000 ppm to producepre-treated water;

a Capacitive De-Ionisation (CDI) assembly comprising at least oneflowpath for pre-treated water arranged between a pair of substantiallyparallel oppositely charged electrodes which adsorb and thereby removeat least some ions from the pre-treated water flowing through theflowpath, for producing treated water with a reduced salinity and solidscontent; andmeans for injecting the treated water with reduced salinity and solidscontent into the subsurface formation to mobilize crude oil and therebyenhance crude oil recovery from the formation.

The filtration assembly may comprise a capillary Nano Filtration (NF)and/or microfilter assembly and may be configured to generatepre-treated water with a reduced hardness and sulphate concentration andwith less than 1 parts per million (ppm) oil and less than 1 parts permillion (ppm) of Total Suspended Solids (TSS).

These and other features, embodiments and advantages of the methodand/or system according to the invention are described in theaccompanying claims, example, abstract and the following detaileddescription of non-limiting embodiments depicted in the accompanyingdrawings, in which description reference numerals are used which referto corresponding reference numerals that are depicted in the drawings.

Similar reference numerals in different figures denote the same orsimilar objects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a process scheme of filtration and CapacitiveDe-Ionisation (CDI) assemblies for producing treated water suitable forEOR in accordance with the invention; and

FIG. 2 shows in more detail a longitudinal sectional view of a flowchannel in the CDI assembly shown in FIG. 1 in which ions are removedfrom saline water.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

FIG. 1 depicts a process scheme for an EOR injection water treatmentfacility according to the invention.

This EOR injection water treatment facility comprises a solids removalfilter 1, a capillary Nano-Filtration (NF) unit 2 and a CapacitiveDe-Ionisation (CDI) unit 3.

A stream 4 of raw source water, which is optionally blended with oralternated by a stream of recycled injection and pore water from theformation, may be fed to the solids removal filter in which coarseparticles are removed and the stream 4 may be split into a firstpre-treated water stream 5 and a first reject water stream 6. The firstpre-treated water stream may be subsequently fed into the capillary ornon-capillary Nano-Filtration (NF) unit 2, in which the firstpre-treated water stream may be split into a second reject water stream7 and a second pre-treated water stream 8.

The second pre-treated water stream 8 may be then fed to the CapacitiveDe-Ionisation (CDI) unit 3 in which the second pre-treated water stream8 may be split into a third reject water stream 9 and a treated EORinjection water stream 10.

Reducing hardness and salinity of treated injection water 10 offersopportunities to Enhance Oil Recovery (EOR).

Mixing or alternating the flux of the source water 4 with low-salinitybrine 10 is feasible if originally highly saline formation water, richin divalent ions, is present in the pores of the reservoir rock of theoil bearing formation. The presence of clays in reservoir rock dictatesa lower Total Dissolved Solids (TDS). Typical TDS limits are 1000-5000parts per million (ppm). Injection of fresh water for EOR would lead toformation damage from clay swelling.

It is known from UK patent GB2450269 that desalination of water andremoval of hardness is also very effective in (i) reducing the amountsof polymers and surfactants required for chemical EOR, and (ii) reducingthe risks of reservoir souring and formation of scale.

Sources of injection water may be seawater, brackish water, aquiferwater, or produced water, the selection of which depends for example onthe location of the oilfield, environmental discharge limits and/ortargets to re-use produced water.

Conventional seawater desalination technologies can be classified asdistillation methods (e.g. MSF, MED) and membrane processes, such as RO(Reverse Osmosis), NF (Nano Filtration) and Electrodialyses.

RO, such as SWRO (=Sea Water Reverse Osmosis) with either conventionalor membrane pre-treatment is presently the most viable desalinationmethod available for offshore application where space is generallyconstraint.

A drawback of RO, however, is that: (i) permeate water of a too highpurity is produced and blending with a higher salinity feedwater isrequired to achieve the required TDS level, and (ii) RO is sensitive tofouling, and (iii) energy-intensive.

In accordance with the invention an alternative solution is provided forthe removal of TDS that overcomes the drawbacks of (SW) RO, i.e. theapplication of CDI (Capacitive De-Ionisation), possibly combined withcapillary NF.

An advantage of using a CDI unit 3 for treating water used for EOR isthat the product salinity/hardness can be tuned by the charge of theelectrodes; hence blending to increase TDS again, as illustrated byarrows 11 would not be required.

Moreover, no additional chemicals are needed for the regeneration of theCDI unit 3.

FIG. 2 shows that a CDI unit 3 having an open flowpath 12 arrangedbetween substantially parallel electrode assemblies comprisingpositively and negatively charged electrodes 13 and 14, which arecovered by upper and lower porous carbon electrodes 14 and 15. The upperporous carbon electrode 14 may be covered by an anion exchange membrane17 and the lower porous carbon electrode 16 may be covered by a cationexchange membrane 18.

The stream of pre-treated water 8 discharged by the NF unit 2 shown inFIG. 1 may flow through the flowpath 12 between the positively andnegatively charged electrodes 13 and 14 which attract cations 19 andanions 20, respectively, thereby causing migration of cations into thepores of the upper carbon electrode 15 and migration of anions into thelower carbon electrode 16.

The absence of flow barriers in the open flowpath 12 between the anionand cation exchange membranes 17 and 18 may significantly reduce therisk of fouling in comparison with a RO membrane, which is prone tofouling due to the flow of water through a fine mesh of openings in thewall of the RO membrane. Also no high pressure pumps, membranes,distillation columns or thermal heaters are required.

Pre-treatment may be required as illustrated in FIG. 1 to preventclogging of a carbon cloth that may be used in the porous carbonelectrodes 15 and 16 in the CDI unit 3, for example by pre-treating theseawater stream 4 with a micro and/or other filter 1 and/or a capillaryor non-capillary Nano-Filtration (NF) unit 2 shown in FIG. 1.

It has been found that the presence of Natural Organic Matter (NOM) inthe source water appears to reduce the inorganic sorption capacity ofthe carbon aerogel material. Pretreatment for NOM removal may aid theoperational efficiency of the CDI process using carbon aerogels.

Partial removal of divalent ions for reduction of hardness and sulphateconcentration by application of capillary nano-filtration membranes maybe effective as pre-treatment as it is less susceptible to fouling thanspiral wound membranes; the latter makes the solution tolerant for thepresence of oil and solid traces potentially allowing the re-use ofproduced water. Partial removal of the presence of higher valency ionspotentially also allows better process control in CDI as lesspreferential loading can be expected.

In comparison with known MF-SWRO desalination methods, the followingkey-differentiators for the CDI desalination method according to theinvention are envisaged:

-   -   1. The CDI desaliniation method may require less weight and        space; hence the CDI desaliniation method may be applied on        certain offshore oil recovery platforms and vessels that are too        space and/or weight constrained for the application of MF-SWRO.        Also, the lower space and weight requirements may result in        significantly lower costs when applied on an offshore platform        or vessel.    -   2. The CDI desalination method may provide higher process        efficiency because no blending is required.    -   3. The CDI desalination method may result in less fouling        problems, and, therefore, the process may require fewer        remediation measures.    -   4. The CDI desaliniation method may result in less energy        consumption and no chemical consumption.    -   5. The CDI desalination method may provide a higher degree of        operability and maintainability.

It has been observed that EOR water flooding with low salinity (TDS˜3000 ppm; TDS=total dissolved solids) water instead of raw seawaterinjection Improves Oil Recovery (IOR) and is potentiallycost-competitive compared to chemical Enhanced Oil Recovery (EOR)methods. Core flow tests and single well chemical tracer tests haveshown that low-salinity water flooding can improve the hydrocarbonrecovery efficiency by 5 to 38% of Original Oil In Place (OOIP).

Injection of a stream of low salinity water 10 may shift the wettabilityof reservoir rock towards a more water-wet state and hence may result inincremental oil recovery. The reverse effect is also possible. Theeffectiveness of the process is known to depend upon parameters likecomposition of formation water (ion content, pH), initial watersaturation, clay content of the rock formation and oil properties. Also,when used in polymer flooding, the low salinity water stream 10 mayrequire considerably less amounts of polymer, thus reducing thefacilities required offshore for transportation, storage, and handlingof polymer chemicals.

A summary of optional performance of the water treatment facilities 1-3shown in FIG. 1 is provided below:

-   1. Pre-treatment in the NF/RO (membrane softening & desalination)    units 1 and 2:    -   Oil <1 ppm    -   Total suspended solids TSS <1 ppm-   2. Low salinity waterflooding stream 10:    -   Salt: TDS-1000-5000 ppm    -   low hardness to limit amount of an optionally added EOR and/or        viscosifying polymer-   3. Treated EOR water stream 10, if used as feed for an ASP (Alkaline    Surfactant Polymer) cocktail:    -   Salt: TDS-1000-2000 ppm    -   De-aerate: O₂<20 ppb    -   Iron: Fe<2 ppm    -   min. O₂ & Fe levels are required to prevent polymer        degradation/precipitation-   4. Scaling (hardness)/Souring of treated EOR water stream 10:    -   Softening: Ca<40 ppm, Mg<100 ppm,    -   SRU (Sulphate Removal Unit) against souring: SO4<20 ppm.        -   A Sulphate Removal Unit (SRU) may comprise Nano-Filtration            (NF) membranes to remove multi or divalent anions from            water, such as SO₄ ²⁻. In the presence of SRB (Sulphate            Reducing Bacteria) sulphate is converted to HS—, leading to            souring of the reservoir.            It is known from International patent application WO            2011/135048 that Capacitive Deionization (CDI) is a            desalination technology based on ion accumulation into an            electric double layer. This double layer is formed when an            electrically charged surface of the porous carbon electrode            15,16 is introduced into an aqueous electrolyte solution            provided by the pre-treated saline water stream 8. The            amount of charge used for double layer formation is directly            proportional to the amount of ions 19,20 which can be            removed. CDI competes with RO, Ion-Exchange and            Electrodialyses but unlike some of these conventional            processes no additional chemicals are needed for the            regeneration of the unit. Also no high pressure pumps,            membranes, distillation columns or thermal heaters are            required. The principle of CDI originates from the 1970's.            In those days however no suitable materials having high            surface area and low electrical resistance were cheaply            available, therefore it was not yet feasible to apply this            technology to desalination of a saline water stream 8.            Nowadays, materials are getting available at lower prices,            such as (extruded) powder, fibers and nanotubes. To achieve            the maximum amount of adsorption capacity in a short period            of time, high surface area materials are used with low            electrical resistance, such as activated carbons for making            the porous carbon electrodes 15 and 16. Most of present CDI            research uses carbon aerogel as adsorbing material in the            porous carbon electrodes 15 and 16.            In a competitive environment (i.e., when multiple ions of            varying valences are present), the sorption of the divalent            species in the porous carbon electrodes 15 and 16 is            limited.            The pre-treated water stream 6 may flow through the open            flowpath 12 between the cation and anion exchange membranes            17 and 18 covering the positively and negatively charged            porous carbon electrodes 15 and 16.            By creating a pre-determined electrical potential between            the positively and negatively charged carbon electrodes 15            and 16, for example by applying a positive voltage to the            current positively charged collector of the first electrode            (the anode) 13 and a negative voltage to the current            collector of the second electrode (cathode) 15, the anions            of the pre-treated water flowing through the open flowpath            12 are attracted to the negatively charged porous electrode            16 and the cations are attracted to the positively charged            porous electrode 15. In this way the ions, comprising            cations 19 and anions 20, will be removed from the water            flowing through the flowpath 12. When the porous electrodes            15 and 16 are saturated with ions the porous electrodes 15            and 16 may be regenerated by releasing the potential            difference and electrically discharging the porous            electrodes 15 and 16. This will release the ions from the            porous electrodes 15 and 16 into the water flowing through            the flowpath 12. This will result in an increase in the ion            content in the water flowing through the flowpath 12 and            this water will be flushed out of the flowpath 12. Once most            ions are released from the porous electrodes 15 and 16 and            the contaminated water with increased ion content is flushed            out of the flowpath 12 the porous electrodes 15 and 16 are            regenerated and can be used again for attracting ions for            water desalination.            The electrical potential differences between the anode and            the cathode porous electrodes 15 and 16 are rather low, for            example less than 2 Volt, preferably less than 1.7 Volt and            even more preferably less than 1.4 Volt.            It is also useful if the electrical resistance of the            electrical circuit provided by the current collectors 13,14            and the porous electrodes 15,16 is low.            The carbon used in the porous carbon electrodes 15 and 16            may comprise activated carbon, and optionally any other            carbonaceous material, such as carbon black, carbon            aero-gels, carbon nanofibres graphene or carbon nanotubes.            The carbon may be chemically activated carbon or may be            steam activated carbon. The carbon may have a high surface            area of at least 500 m²/g, preferably at least 1000 m²/g,            and more preferable at least 1500 m²/g. The cathodic and            anodic current collectors 13 and 14 may even be made out of            different carbonaceous materials. The porous carbon            electrodes 15 and 16 may comprise non-flexible carbon layers            comprising carbon aerogels. These aerogels may be            manufactured as composite paper: non-woven paper made of            carbon fibers, impregnated with resorcinol-formaldehyde            aerogel, and pyrolyzed. Depending on the density, carbon            aerogels may be electrically conductive, making composite            aerogel paper useful for electrodes in capacitors or            deionization electrodes.            The carbon may be present in the porous electrodes 15 and 16            in a concentration of at least 60%, preferably at least 70%,            more preferable at least 80%, or even at least 85% by weight            of the dry electrode. The use of thermoplastic or            viscoelastic material such as latex or curable resins to            form monoliths from powdered material is common Examples of            carbon layers that use polyfluorotetraethylene (PTFE) as            binder material are the PACMM™ series commercially available            from Material Methods LLC, 30 Hughes, Suite 205, Irvine,            Calif. 92618, USA.            The CDI unit 3 according to the invention may comprise            porous carbon electrodes 15 and 16 comprising active carbon            fiber woven layer or carbon cloth, e.g. ZORFLEX®            commercially available from Chemviron Carbon, Zoning            Industriel C de Feluy, B-7181 Feluy, Belgium.            Alternatively the CDI unit 3 according to the invention may            comprise porous carbon electrodes 15 and 16 covered by anion            and cation exchange membranes 17 and 18 comprising a carbon            coating comprising polyelectrolyte a binder and carbon,            which may be coated directly onto the current collector.            The current collectors 13 and 14 may be made from any            suitable metal or metal free electrically conducting            material. Suitable metal free materials are e.g. carbon,            such as graphite, graphene, graphite sheets or carbon            mixtures with high graphite content. It is advantageous to            use a metal free electrode because metals are expensive and            introduce a risk of corrosion. The current collector is            generally in the form of a sheet. Such sheet is herein            defined to be suitable to transport at least 33 Amps/m² and            up to 2000 Amps/m². The thickness of a graphite current            collector 13, 14 then typically becomes from 100 to 1000            micrometer, generally 200 to 500 micrometer.            The flow path 12 may comprise a spacer made of a permeable            inert type material such as an open space synthetic            material, plastic and/or fiberglass.            The spacer may be made of a material that is electrically            insulating, but allows ion conductance. Suitable spacers are            for example the NITEX® or PETEX® commercially available from            Sefar Inc., 111 Calumet Street, Depew, N.Y. 14043, USA,            which are open mesh fabrics or filter fabrics, made from            polyamide or polyethyleneterephthalte.            The anion and cation exchange membranes 17 and 18 may            comprise a charge barrier, which is selective for anions or            cations, or certain specific anions or cations, and which            may be placed between the porous electrodes 15 and 16 and            the spacer in the flow path 12. The charge barrier may be            applied to the high surface area electrode layer as a            coating layer or as a laminate layer.            Suitable membrane materials may be homogeneous or            heterogeneous. Suitable membrane materials comprise anion            exchange and/or cation exchange membrane materials,            preferably ion exchange materials comprising strongly            dissociating anionic groups and/or strongly dissociating            cationic groups. Examples of such membrane materials are            NEOSEPTA™ commercially available from Tokuyama Corp., PC-SA™            and PC-SK™ commercially available from PCA GmbH, ion            exchange membrane materials commercially available from            FuMA-Tech GmbH, ion exchange membrane materials RALEX™            commercially available from Mega or EXCELLION™ heterogeneous            membrane material commercially available from Snowpure.            The porous electrodes 15 and 16 may be floating electrodes            that are not directly connected to a power source but            receive their charge from other electrodes in a stack of CDI            units 3 which are connected to an electrical power source.            Floating electrodes may be positioned parallel and in            between the master electrodes in a stack of CDI units 3. An            advantage of using floating electrodes is that the voltages            through the CDI unit 3 will be higher while the currents            through CDI unit 3 will be lower. Electrical resistivity in            the CDI unit 3 may be lowered significantly by using            floating electrodes.

EXAMPLE

In this example three different processes for seawater desalination arecompared, see table 1. In all three processes a pre-treatment step usinga high-performance nanofiltration step is applied to produce a permeatewhich usually has a TDS equivalent to brackish water.

TABLE 1 Comparison of three membrane processes for seawater desalinationStage 1 Stage 2 Process 1 high-performance Brackish water reversedosmosis nanofiltration (BWRO) membranes Process 2 high-performancehigh-performance nanofiltration nanofiltration Process 3high-performance Capacitive Deionisation (CDI) nanofiltration

Process 1 is a commonly applied method to produce potable water fromseawater based on nano-filtration and reversed osmosis.

Process 2 is a known dual stage Nano Filtration (NF) seawaterdesalination system described in U.S. Pat. No. 7,144,511 by Diem XuanVuong. A disadvantage of this known dual stage NF process is its loweroverall recovery rate compared to process 1.

Process 3 describes the combination of Nano-Filtration with CapacitiveDe-Ionization (CDI) in accordance with the invention.

In tables 2,3 and 4 below, performance data for the three processes arepresented.

It can be seen that the known Processes 1 and 2 produce a permeatequality that does not meet the required TDS specifications for IOR orEOR purposes (TDS=62 and 586 mg/L, respectively).

In order to obtain the desired water quality blending with raw seawateris required.

In contrast Process 3 according to the invention is able to produce thetarget water specifications without the need for blending, resulting insignificantly lower operating cost.

TABLE 2 Performance data for Process 1 (Prior art: Seawater desalinationusing a NF-BWRO) Permeate Permeate Seawater from first from 2ndcomposition stage NF stage BWRO mg/L mg/L mg/L Ca2+ 410 16 0.1 Mg2+ 131052 0.5 Na+ 10900 1635 24 K+ 390 62 1 Ba2+ 0.05 0 0 Sr2+ 13 0 0 Fe3+ 0.020 0 Mn2+ 0.01 0 0 SiO4 8 0 0 Cl− 19700 2758 36 SO2− 2740 55 0.3 F− 1.4 00 HCO3− 152 8 0.2 TDS 35624 4587 62

TABLE 3 Performance data for Process 2 (Prior Art: Seawater desalinationusing a NF-NF line-up) Seawater Permeate from first Permeate fromcomposition stage NF 2nd stage NF mg/L mg/L mg/L Ca2+ 410 16 0.9 Mg2+1310 52 3 Na+ 10900 1635 218 K+ 390 62 9 Ba2+ 0.05 0 0 Sr2+ 13 0 0 Fe3+0.02 0 0 Mn2+ 0.01 0 0 SiO4 8 0 0 Cl− 19700 2758 353 SO2− 2740 55 1 F−1.4 0 0 HCO3− 152 8 0.6 TDS 35624 4587 586

TABLE 4 Performance data for Process 3 (Seawater desalination using aNF-CDI line-up in accordance with the present invention) SeawaterPermeate from first Permeate from composition stage NF CDI mg/L mg/Lmg/L Ca2+ 410 16 8.2 Mg2+ 1310 52 26.2 Na+ 10900 1635 817.5 K+ 390 6231.2 Ba2+ 0.05 0 0.0005 Sr2+ 13 0 0.195 Fe3+ 0.02 0 0.0002 Mn2+ 0.01 00.0001 SiO4 8 0 0.04 Cl− 19700 2758 1379 SO2− 2740 55 27.4 F− 1.4 00.098 HCO3− 152 8 3.8 TDS 35624 4587 2294The present invention is well adapted to attain the ends and advantagesmentioned as well as those that are inherent therein. The particularembodiments disclosed above are illustrative only, as the presentinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. While systems and methods are described in terms of“comprising,” “containing,” or “including” various components or steps,the compositions and methods can also “consist essentially of” or“consist of” the various components and steps. Whenever a numericalrange with a lower limit and an upper limit is disclosed, any number andany included range falling within the range is specifically disclosed.In particular, every range of values (of the form, “from a to b,” or,equivalently, “from a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues. Whenever a numerical range having a specific lower limit only, aspecific upper limit only, or a specific upper limit and a specificlower limit is disclosed, the range also includes any numerical value“about” the specified lower limit and/or the specified upper limit.Also, the terms in the claims have their plain, ordinary meaning unlessotherwise explicitly and clearly defined by the patentee. Moreover, theindefinite articles “a” or “an”, as used in the claims, are definedherein to mean one or more than one of the element that it introduces.

What is claimed is:
 1. A method for enhancing oil recovery from an oilbearing formation, the method comprising: filtering at least some solidsfrom a source water having a total dissolved solids content of from10000 ppm to 50000 ppm in a filtration assembly to produce pre-treatedwater; treating the pre-treated water in a capacitive de-ionisationassembly comprising at least one flowpath for pre-treated water arrangedbetween a pair of oppositely charged electrodes which adsorb and therebyremove at least some ions from the pre-treated water flowing through theflowpath, thereby producing treated water having a total dissolvedsolids (TDS) content between 1,000 and 5,000 parts per million (ppm);and injecting the treated water with reduced salinity and solids contentinto the oil bearing formation to mobilize crude oil and enhance oilrecovery.
 2. The method of claim 1, wherein the electrodes comprisesubstantially parallel porous plates comprising activated carbons thatare electrically charged by a direct current electrical power sourceconnected to the plates.
 3. The method of claim 2, wherein the porousplates comprise carbon aerogel.
 4. The method of claim 1, wherein thefiltration assembly comprises a microfilter assembly.
 5. The method ofclaim 1, wherein the filtration assembly generates pre-treated waterwith a reduced hardness and sulphate concentration relative to thesource water and with less than 1 parts per million (ppm) oil and lessthan 1 part per million (ppm) total suspended solids.
 6. The method ofclaim 1, wherein the source water has TDS content between 30,000 and40,000 mg/liter.
 7. The method of claim 6, wherein the treated water hasa TDS between 2,000 and 5,000 parts per million (ppm).
 8. The method ofclaim 7, wherein the treated water has a lower salinity than pore waterpresent in the formation before initiation of treated water injection.9. The method of claim 7, wherein the treated water has a lower ionicstrength than pore water present in the formation before initiation oftreated water injection.
 10. The method of claim 1 wherein the sourcewater is selected from the group consisting of seawater, brackish water,water produced from the oil bearing formation, saline aquifer water, andmixtures thereof.